In motor vehicles the performance of occupant protection devices, e.g. of force limiters used in belt retractors, is increasingly adapted depending on the seriousness of an accident and the position of the occupant. For this purpose the information as to the relative or absolute belt webbing unwinding is very helpful.
In belt retractors in which a retractor drive spring for winding up the belt webbing is supported by a motor to ensure an increased comfort in the fastened state, it is likewise necessary to recognize the belt webbing unwinding or the belt webbing position to detect whether the belt is in a fastened position, in which it does not move and abuts at a vehicle occupant, or moves at all. The assisting motor may then be activated as required.
To determine the belt webbing unwinding in a belt retractor, systems are known, in which a control disk of a child safety system is provided with a signal transmitter so that the rotation of the control disk may be detected via an associated sensor. During the normal driving operation the control disk is coupled to the belt reel via a reduction gear such that the complete belt webbing unwinding is mapped to approximately one rotation of the control disk. In this manner the belt reel rotations and thus the belt webbing unwinding may be detected.
The invention creates a belt retractor for a safety belt system and a method of determining the belt webbing unwinding, by means of which the relative belt webbing unwinding may be determined by simple means in a restraint case.